1. Field of the Invention
The present invention relates to a method for recording information on an information-recording medium capable of recording the information by irradiating a laser beam onto the information-recording medium, and the information-recording medium to be used therefor. In particular, the present invention relates to an information-recording method which makes it possible to secure the compatibility of recording between information-recording media and apparatuses in which rising times and falling times of laser powers are different from each other, and the information-recording media to be used therefor. Further, the present invention-relates to a light power control method and an information-recording method which make it possible to easily optimize laser powers for apparatuses having different recording linear velocities, and an information-recording medium to be used therefor.
2. Description of the Related Art
In recent years, the market of read-only optical disks including, for example, DVD-ROM and DVD-Video is expanded. On the other hand, rewritable DVD's including, for example, DVD-RAM, DVD-RW, and DVD-+RW are introduced into the market, and the market thereof is expanding for backup media for computers and image-recording media with which VTR may be substituted. In these several years, the demand of the market has increased for the improvement in the access speed and the transfer rate of recordable DVD's.
The method for recording information on the optical disk includes the CLV (Constant Linear Velocity) system and the CAV (Constant Angular velocity) system. The CLV system resides in such a control method that the number of revolutions of the optical disk, i.e., the relative velocity between the laser beam and the optical disk is constant. On the other hand, the CAV system resides in such a system that the angular velocity, which is used to rotate the optical disk, is made constant to control the rotation.
The CLV system has the following features. (1) The signal processing circuit can be extremely simplified, because the data transfer rate is always constant during the recording and reproduction. (2) When the laser beam is moved in the radial direction of the optical disk, it is necessary that the number of revolutions of the motor is controlled again depending on the radial position. Therefore, the access speed is greatly lowered.
The CAV system has the following features. (1) The signal processing circuit is large-sized, because the data transfer rate differs depending on the radial position during the recording and reproduction. (2) It is possible to effect the high speed access, because it is unnecessary that the number of revolutions of the motor is controlled again depending on the radial position when the laser beam is moved in the radial direction of the optical disk.
The phase-change recording system is adopted for recordable DVD media such as DVD-RAM and DVD-RW on which information is recordable and erasable. In the case of the phase-change recording system, the recording is basically performed such that pieces of information of “0” and “1” are allowed to correspond to the crystal and the amorphous. Recorded “0” and “1” can be detected by radiating the laser beam to the crystallized portion and the amorphous portion and effecting the reproduction based on the reflected light beam.
In order to bring about the amorphous state at a predetermined position, the heating is effected so that the temperature of the recording layer is not less than the melting point of the recording layer material by radiating a laser beam having a relatively high power. On the other hand, in order to bring about the crystalline state at a predetermined position, the heating is effected so that the temperature of the recording layer is in the vicinity of the crystallization temperature of not more than the melting point of the recording layer material by radiating a laser beam having a relatively low power. By dosing so, it is possible to reversibly change the amorphous state and the crystalline state.
A phenomenon called “recrystallization” occurs in the phase-change recording, in which the crystal growth takes place from the outer edge of the melted area during the cooling process which occurs immediately after the heating of the recording layer material to the temperature of not less than the melting point by means of the laser beam, and the size of the recording mark is consequently decreased. In order to suppress the deterioration of the shape of the recording mark caused by the recrystallization, the following method is generally adopted as described, for example, in Japanese Patent Application Laid-open Nos. 62-259229 and 3-185629. That is, the recording power is not radiated in a direct current manner, but the power is once lowered after the radiation of the recording power so that the radiation is effected in a form of pulse sequence. The design or construction of the recording pulse sequence is referred to as “recording strategy”.
The optimization of the recording power may be explained as exemplified by a drive for DVD-RAM. That is, the data is subjected to the trial writing by using a value of the recording power written on the disk, and the recording power is finely adjusted so that the error rate in the trial writing data is minimized. Accordingly, the recording power is optimized.
In the case of the rewritable information-recording medium such as the optical disk, it is extremely important to secure the compatibility or the interchangeability with respect to the information-recording apparatuses based on various standards and produced by various manufacturers. For example, the DVD-RAM. medium may be exemplified as follows. A DVD-RAM drive, which is adapted to the ×2 speed based on the CLV rotation control (data transfer rate: 22 Mbps, linear velocity: 8.2 m/s), is already present in the market. However, in order to satisfy the demand of the market for the improvement in the transfer rate and the access speed, it is considered that those dominantly used in future may be drives adapted to CLV in which the recording linear velocity is enhanced and drives adapted to CAV. Therefore, it is extremely important and indispensable for the benefit of consumers to guarantee the compatibility of the recording between the drive adapted to CAV and the drive adapted to the CLV in which the recording linear velocity and the transfer rate are different from each other, or between the drives of which the laser light sources are different from each other in the response characteristic.
However, it is necessary to increase the frequency of the recording signal as the recording linear velocity is increased and the data transfer rate is raised. When the laser beam is pulse-modulated and radiated as described above, the time width of each of the pulses for constructing the pulse sequence is extremely shortened. On the other hand, the laser light-emitting element requires a certain period of time from the application of the driving current to the arrival of the light emission intensity at the intensity corresponding to the concerning current value. Therefore, if the width of the pulse becomes shorter than the period of time which is required until the light emission intensity of the light-emitting element arrives at the intensity corresponding to the driving current value in order to enhance the transfer rate, the laser light emission corresponding to each pulse is attenuated before the arrival at the peak value. As a result, the energy per unit area, which is applied to the recording medium by the laser power, is deviated from the optimum value. Further, the shape of the recording mark to be written on the recording medium is distorted, and it is consequently impossible to record and reproduce information correctly. In particular, the following problem arises. That is, when the linear velocity is increased, it is impossible to reliably record information in the case of the conventional recording strategy which uses any conventional pulse sequence.
On the other hand, the period of time, in which the light emission intensity arrives at the intensity corresponding to the current value after the driving current for the light-emitting element is applied, greatly differs depending on the type of the light-emitting element carried on the information-recording apparatus even when the wavelength for the laser light-emitting element is identical. Therefore, even when the recording is performed in accordance with an identical strategy, the energy per unit area, which is applied to the recording, is deviated from the optimum value depending on the type of the light-emitting element. Further, the shape of the recording mark to be written on the recording medium is distorted, and it is consequently impossible to record and reproduce information correctly. That is, there is no guarantee to record any identical recording mark because of the difference in the performance of the recording apparatus to be used, especially in the response characteristic of the laser light source even when any identical recording strategy is used.
The setting of the power is extremely important when information is recorded on the recording medium as described above. However, the situation is varied in a complicated manner depending on the unsaturation phenomenon of the laser power due to the recording linear velocity as well as on the difference in the rising time and the falling time of the laser power depending on the type of the light-emitting element. Therefore, it is not easy to establish the optimum power for the information-recording apparatus.